Carbon black has so far been used as a reinforcing filler for rubber. This is because carbon black can provide rubber compositions with high abrasion resistance. In recent years, as resource saving and energy saving are socially requested, low heat build-up of tire rubber has come to be requested as well at the same time for the purpose of saving fuel consumption of cars. When low heat build-up is intended to be achieved by using carbon black alone, it is consider that a compounding amount of carbon black is reduced or that carbon black having a large particle diameter is used, but it is known that in both cases, the reinforcing property, the abrasion resistance and the grip on a wet road are not prevented from being reduced. On the other hand, it is known to use silica as a filler in order to enhance low heat build-up (refer to, for example, patent documents 1 to 4). However, particles of silica tend to be coagulated by virtue of a hydrogen bond of a silanol group which is a surface functional group of silica, and a silanol group is not good in wettability with a rubber molecule due to a —OH group having hydrophilicity to deteriorate dispersion of silica into rubber. The kneading time has to be extended in order to improve the above problem. Also, a Mooney viscosity of a rubber composition is elevated due to insufficient dispersion of silica into rubber, and the defect that the rubber composition is inferior in processability such as extrusion has been involved therein. Further, since a surface of a silica particle is acidic, silica adsorbs a basic substance used as a vulcanization accelerator in vulcanizing a rubber composition to prevent the rubber composition from being sufficiently vulcanized, so that the defect that the elastic modulus is not enhanced has been involved therein as well.
A silane coupling agent has been developed in order to improve the above defects, but dispersion of silica has not yet reached a sufficiently high level, and it has been particularly difficult to obtain industrially good dispersion of silica particles. Accordingly, it is tried to mix silica whose surface is treated with a hydrophobicity-providing agent to accelerate reaction of a silane coupling agent (refer to, for example, patent document 1).
Further, it is disclosed in patent document 5 to use hydrophobic precipitated silica. However, since precipitated silica subjected to complete hydrophobicity treatment is used, surface silanol groups to react with a silane coupling agent disappear, and therefore the defect that rubber is not sufficiently reinforced has been involved therein. Further, silica having an increased particle diameter is used in order to improve low heat build-up, but silica is reduced in a specific surface area by increasing a particle diameter to deteriorate a reinforcing property thereof. It is disclosed in patent document 6 to use silica having a specific form, but low heat build-up and abrasion resistance of the rubber composition are not sufficiently high.
On the other hand, requirement to reduction in fuel consumption of cars is being increased in recent years, and tires having small rolling resistance are required. Accordingly, rubber compositions which have low tan δ (hereinafter referred to as a low hysteresis) and which are excellent in low heat build-up are required as rubber compositions used for a tread and the like in tires. Further, rubber compositions used for a tread are required to be excellent in abrasion resistance and a breaking characteristic in addition to a low hysteresis from the viewpoints of the safety and the economical efficiency. In contrast with this, in order to improve a low hysteresis, abrasion resistance and a breaking characteristic of a rubber composition prepared by compounding a rubber component with a reinforcing filler such as carbon black, silica, it is effective to enhance affinity between the reinforcing filler and the rubber component in the rubber composition.
For example, synthetic rubbers which are improved in affinity with a reinforcing filler by modification of a polymer end (refer to, for example, patent documents 7, 8, 9, 10 and 11) and synthetic rubbers which are improved in affinity with a reinforcing filler by modification of the main chain (refer to, for example, patent documents 12 and 13) are developed in order to enhance affinity between a reinforcing filler and a rubber component in a rubber composition to improve the reinforcing effect by the reinforcing filler.
However, the present inventors have studied low heat build-up, abrasion resistance and a breaking characteristic of rubber compositions prepared by using the modified synthetic rubbers described in the patent documents 7 to 13 described above to find that while the above rubber compositions are excellent in low heat build-up, abrasion resistance and a breaking characteristic as compared with those of rubber compositions prepared by using conventional synthetic rubbers, they are not necessarily satisfactory and that they still have room to be improved.    Patent document 1: Japanese Patent Application Laid-Open Hei 6 No. 248116    Patent document 2: Japanese Patent Application Laid-Open Hei 7 No. 70369    Patent document 3: Japanese Patent Application Laid-Open Hei 8 No. 245838    Patent document 4: Japanese Patent Application Laid-Open Hei 3 No. 252431    Patent document 5: Japanese Patent Application Laid-Open Hei 6 No. 157825    Patent document 6: Japanese Patent Application Laid-Open No. 2006-37046    Patent document 7: International Publication Laid-Open No. 2003-046020 pamphlet    Patent document 8: Publication of Japanese Translation of PCT International Publication Application No. 2004-513987    Patent document 9: Japanese Patent Application Laid-Open Hei 11 No. 29603    Patent document 10: Japanese Patent Application Laid-Open No. 2003-113202    Patent document 11: Japanese Patent Publication Hei 6 No. 29338    Patent document 12: Publication of Japanese Translation of PCT International Application No. 2003-534426    Patent document 13: Japanese Patent Application Laid-Open No. 2002-201310